Date: Oct 06, 2011 Author: Meghan Kerry Source: DesignNews (
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Mobile robots have been designed to travel through water, across rugged outdoor terrains, and even along the surface of glass for automated window cleaning. But can you imagine a robot designed to travel on the surface of a human heart?
Nicholas Patronik, Ph.D., and Peter Allen at the Carnegie Mellon Robotics Institute helped design a robot capable of doing just that. The HeartLander, a miniature robotic device, crawls around the surface of the heart in an inchworm-like fashion, taking measurements and performing simple surgical tasks. This robot addresses two major challenges -- gaining access to the heart without opening the chest and operating on the heart while it is beating.
Several options for tackling these challenges exist today. Thoracoscopic techniques use laparoscopic tools inserted through the chest cavity to operate on the beating heart. Percutaneous transvenous techniques access inner organs through main arteries and veins. For example, a doctor can guide a heart stent through the veins in your thigh to treat blockages, and this procedure is performed on an outpatient basis. These transvenous procedures are easier to recover from, but thoracoscopic techniques offer much more flexibility in the complexity of surgical operations that can be performed.
The HeartLander robot is considered a hybrid of these two approaches in that it can achieve fine control of thoracoscopic techniques while maintaining the ability to be performed on an outpatient basis like the percutaneous transvenous techniques. It adheres to and traverses the heart's surface, the epicardium, providing a tool for precise and stable interaction with the beating heart. Even better, it can access difficult-to-reach locations of the heart, such as the posterior wall of the left ventricle.
The HeartLander is launched on to the surface of the beating heart through a small puncture under the bottom of the sternum. The robot adheres to the epicardial surface of the heart and navigates autonomously to the specified location, traversing the epicardium like an inchworm. It can navigate to any location on the epicardium with speeds of up to 4mm per second and acquire localization targets within 1mm. The HeartLander's mobility is semi-autonomous. It uses a pure pursuit-tracking algorithm to navigate to predetermined surface targets. It can also be controlled via teleoperation.
Several prototypes of the HeartLander system have been completed. One of the first prototypes used a flexible tether with offboard linear motors to actuate the robot forward while solenoids regulated vacuum pressure to suction pads.
The tethered design offloads functional components of the crawler, making it simpler and robust, but a tetherless robot was desired to reduce tether stiffness, thereby increasing turning efficiency. While studying at the University of Pittsburgh, Allen made some of the first steps toward a tetherless robot and designed a robot with onboard motors.
His first prototype of a robot with onboard motors was the length of a quarter and no wider than the length of your pinky finger. It was a challenge to find tiny motors that would push the robot on the heart, but Allen was able to use two ultrasonic piezoelectric linear motors, each capable of producing an output force of 1N. Locomotion is accomplished in the same general manner as the other HeartLander prototypes -- using alternating inchworm-style locomotion with suction on each of the two feet to adhere to the epicardium.
By using the NI LabVIEW graphical development environment and an NI Data Acquisition device, Allen says, he was able to prototype the tetherless robot quickly and design a software program for motor control and path planning. The motor control portion of the application controls different stepping and vacuum motions, and the control system uses tracking data from an onboard magnetic sensor to provide autonomous navigation to the specified target.
After the target has been acquired, the physician controls locomotion and therapy using a joystick and a LabVIEW graphical interface that shows the exact location of the robot on the heart. The real-time location is measured using a miniature magnetic tracking sensor (microBIRD, Ascension Technology) located on the front body of the crawling robot. The robot can be driven using the joystick, or it can walk automatically to a specified target location on the heart.
The crawling robot also contains a 2mm working port through which tools can be deployed for a variety of epicardial interventions. Thus far, dye injections and epicardial pacing lead placement have been performed percutaneously using the HeartLander. In the future, the front module will be equipped with modular end-effectors for more innovative therapeutic applications.
Allen's tetherless prototype has demonstrated locomotion on the anthropomorphic heart phantom. However, there is a tradeoff between onboard and offboard motor prototypes. The need to mount the motors within the crawler makes miniaturization more challenging -- a difficulty that must be addressed in light of the spatial constraints of the intrapericardial environment. As a result, the onboard motor prototype is considered for future development and miniaturization, but not for live animal testing.
The current design is a crawling robot with two body sections (front and rear) that are each 5mm tall, 8mm wide, and 10mm long. The inchworm-like locomotion of the crawling robot is generated by alternating the suction to the body sections while changing the distance between them. Suction is controlled by pressure sensors and electronic valves as the distance between the bodies is controlled by external linear actuators that push and pull the drive wires running through the tether. The computer coordinates the actions and timing of the locomotion, which are transparent to the surgeon. Finally, the angle of the front body can be adjusted to steer or adapt to the curvature of the heart. This design produces a robot that is small, lightweight, and disposable.
The novelty of the HeartLander is that it provides a single device for stable and localized sensing, mapping, and treatment over the entire surface of the heart. Additionally, it reduces the damage necessary to access the heart. We don't have inchworm-like robots performing surgery on individuals just yet, but the development of this robot is the first step in designing a wireless mobile robot for cardiac therapy.
